The human body is not well designed for sitting for prolonged periods, yet modern work patterns and lifestyles require us to do just that. When seated, most of the body weight is supported on two pointed bones which form the base of the pelvis. These bones are known as the ischial tuberosities. In the seated state, the pelvis is inherently unstable—it is rather like a triangle balanced on its point.
When seated, the weight of the body trunk, supported by the spine, tends to rotate the pelvis backwards, pushing the spine into a c-shaped curve known as kyphosis. The buttocks then tend to slide forward on the seat, reinforcing the c-curve in the spine.
If the spine is in kyphosis, pressures within the discs of the spine increase very markedly, which will lead to degenerative changes over time, potentially causing severe pain. In addition, with the spine in kyphosis, the rib cage cannot readily expand, nor can the diaphragm move downwards fully. Hence, respiratory efficiency is greatly reduced, which in turn affects many body functions dependent upon blood oxygen levels.
Good posture is important because it minimizes the risk of back pain and improves respiratory efficiency. Various approaches to chair design have been adopted with the aim of encouraging correct seating posture.
These existing designs include the “kneel” chair, popular in the 1970's and 1980's, which supports the buttocks on a surface inclined forward at approximately 30 degrees and prevents the user sliding forward by taking part of the body weight on a knee pad in front of and below the main seating surface. This chair attempts to encourage sound posture initially, but imposes significant load on the knees, which can lead to pain over time. In addition, because no support is provided for the trunk, the user's muscles are constantly in use to stabilize the trunk, which can lead to fatigue over time.
A further example is the “Bambach” saddle chair, which provides a seat member most readily described as a cross between a horse saddle and a bicycle saddle. The user sits astride the seat member. A back support is provided as an option. Like any saddle, this type of seat member causes pressure and chafing and is uncomfortable for females wearing a skirt in particular.
Australian Patent Application No. 73415/87 discloses the “Dual Density” chair. This chair seeks to prevent the pelvis rotating and sliding forward by allowing the ischial tuberosities to become embedded in a soft rear section of the seat cushion and attempts to prevent forward movement by providing a firmer material at the front of the seat, creating a transverse discontinuity, or “split” across the seat. This chair has the disadvantage that the firmer front section of the seat is uncomfortable and exerts pressure on the back of the user's thighs, with possible impedance of peripheral blood circulation in that area.
Further, Australian Patent Application No. 29072/99 discloses the “Soft Cell” chair. This chair seeks to address the disadvantage of the dual density seat in circulatory terms. This is attempted by making the chair seat softer at the front and harder at the back of the seat surface. This chair does not support good posture and performs no differently than a standard foam seat cushion in ergonomic terms.
Another chair design is the “Nottingham” chair, which is designed to allow seating over a wide range of working heights, to allow users of varying stature to address a range of working surface heights comfortably. It attempts to address the issue of spinal stability and correct postural curvature by opening up the angle between the trunk and thighs by encouraging the user to adopt a higher seat height.
Lastly, the “Kneeshaw” chair seeks to maintain pelvic (and hence spinal) alignment by lodging the ischial tuberosities in a depression at the rear edge of the seat cushion.
Accordingly, there is a need to provide a chair that provides good posture and user comfort without the disadvantages of load or pressure on inappropriate areas of the body, such as knees or thighs, and that does not include complex upholstery configurations, such as those which may pose hygiene problems by the accumulation of dirt, for example.
The above statements regarding prior disclosures are not to be taken to be admissions of what was well known in the field of chairs.